Cu(II) complexes derived from N-carboxymethyl and N-carboxyethyl amino acids as catalysts for asymmetric oxidative coupling of 2-naphthol

Article abstract of DOI:10.1016/j.mcat.2019.110480

The synthesis, characterization and catalytic performance of chiral Cu(II) complexes derived from N-carboxymethylated and N-carboxyethylated amino acids is reported. The ligand precursors are prepared by single step N-alkylation of the sodium salts of the appropriate chiral amino acid with either sodium chloroacetate or sodium 3-chloropropionate in water. The Cu(II) complexes are obtained upon reaction of Cu(CH₃COO)₂ with the aqueous or alcoholic suspension of the suitable ligand under vigorous stirring or ultrasound irradiation at room temperature. The Cu(II) compounds are characterised by EPR, UV–vis, circular dichroism and ESI-MS. The molecular structures of two of the prepared complexes are also obtained by single-crystal X-ray diffraction analysis. The catalytic activity of the complexes in the asymmetric oxidative coupling of 2-naphthol is described. All compounds exhibit moderate activity, selectivity and enantioselectivity in ethanol/water mixtures, under aerobic conditions and using potassium iodide as additive. The yields of 1,1′-bi-2-naphthol (BINOL) reached 50% under the optimal conditions, while enantiomeric excesses reached ca. 48%. The effect of variables such as ligand substituents, solvent, temperature and additives on the catalytic activity is also described. In the absence of a base, the complexes only show catalytic activity in the presence of alkali metal iodide such as KI. Details of the oxidative coupling mechanism are studied using spectroscopic and electrochemical methodologies.

Full text of DOI:10.1016/j.mcat.2019.110480

MolecularCatalysis475(2019)110480
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Cu(II) complexes derived from N-carboxymethyl and N-carboxyethyl amino
acids as catalysts for asymmetric oxidative coupling of 2-naphthol
⁎
Pedro Adãoa,b, , Carlos M. Teixeirab, M. Fernanda N.N. Carvalhob, Maxim L. Kuznetsovb,
Clara S.B. Gomesb,c, João Costa Pessoab
a MARE – Marine and Environmental Sciences Centre, ESTM, Instituto Politécnico de Leiria, 2520-641, Peniche, Portugal
b Centro Química Estrutural, Departamento de Engenharia Química, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovísco Pais, 1049-001, Lisboa, Portugal
c LAQV-REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade NOVA de Lisboa, 2829-516, Caparica, Portugal
A R T I C L E I N F O
A B S T R A C T
Keywords:
Amino acid
Copper
2-Naphthol
Oxidative coupling
Asymmetric catalysis
The synthesis, characterization and catalytic performance of chiral Cu(II) complexes derived from N-carbox-
ymethylated and N-carboxyethylated amino acids is reported. The ligand precursors are prepared by single step
N-alkylation of the sodium salts of the appropriate chiral amino acid with either sodium chloroacetate or sodium
3-chloropropionate in water. The Cu(II) complexes are obtained upon reaction of Cu(CH3COO)2 with the aqu-
eous or alcoholic suspension of the suitable ligand under vigorous stirring or ultrasound irradiation at room
temperature. The Cu(II) compounds are characterised by EPR, UV–vis, circular dichroism and ESI-MS. The
molecular structures of two of the prepared complexes are also obtained by single-crystal X-ray diffraction
analysis. The catalytic activity of the complexes in the asymmetric oxidative coupling of 2-naphthol is described.
All compounds exhibit moderate activity, selectivity and enantioselectivity in ethanol/water mixtures, under
aerobic conditions and using potassium iodide as additive. The yields of 1,1′-bi-2-naphthol (BINOL) reached 50%
under the optimal conditions, while enantiomeric excesses reached ca. 48%. The effect of variables such as
ligand substituents, solvent, temperature and additives on the catalytic activity is also described. In the absence
of a base, the complexes only show catalytic activity in the presence of alkali metal iodide such as KI. Details of
the oxidative coupling mechanism are studied using spectroscopic and electrochemical methodologies.
Introduction
of these systems was significant and, despite the many attempts for
optimisation, the overall selectivity towards the formation of BINOL
The synthesis of atropisomeric compounds such as 1,1′-bi-2-naph-
thol (BINOL) by transition metal catalysed aerobic oxidative homo-
coupling under mild conditions remains a challenge [1]. The significant
advances that have been recently made regarding the general subject of
oxidative coupling of phenolic compounds, namely 2-naphthols, reflect
the importance of these synthetic methodologies [2,3].
In light of these recent reports, we continue our effort to develop
structurally simple Cu(II) catalysts for the synthesis of BINOLs, under
the mildest experimental conditions possible.
In a previous work we described a series of bio-inspired copper
complexes derived from N-picolyl-L-amino acids and their catalytic
activity in the asymmetric aerobic oxidative coupling of 2-naphthols, in
aqueous medium [4]. While the catalytic systems did operate under
mild conditions and afforded moderate yields and enantiomeric ex-
cesses for BINOL, the formation of by-products such as naphthoqui-
nones was significant (Scheme 1). The phenol oxygenase character [5]
was moderate at best. Another detrimental aspect was the requirement
for an organic base additive in order to observe catalytic activity. To
develop a more selective copper catalyst we the option of changing
from a [Npy, Namine, OCOO] to a [OCOO, Namine,OCOO] donor atom set.
Preliminary testing of structurally simpler Cu(II) complexes derived
from N-carboxymethyl and N-carboxyethyl-L-phenylalanine indicated
that these were capable of catalysing the asymmetric oxidative coupling
of 2-naphthol under mild conditions. The disadvantage, however, was
that an excess of alkali metal iodide additive was necessary for catalytic
activity in the absence of base additive. The prospect of using simpler
conditions in the synthesis of BINOL prompted the investigation of the
catalytic potential of this series of complexes. Therefore, herein we
report the preparation of Cu-complexes derived from N-carboxymethyl
and/or N-carboxyethyl L-aminoacids (Chart 1 ) and the evaluation of
their catalytic potential in the asymmetric oxidative coupling of 2-
naphthol using air and under mild conditions.
⁎ Corresponding author at: MARE – Marine and Environmental Sciences Centre, ESTM, Instituto Politécnico de Leiria, 2520-641, Peniche, Portugal.
E-mail address: pedro.adao@ipleiria.pt (P. Adão).
https://doi.org/10.1016/j.mcat.2019.110480
Received 10 March 2019; Received in revised form 15 May 2019; Accepted 13 June 2019
2468-8231/©2019ElsevierB.V.Allrightsreserved.

P. Adão, et al.
MolecularCatalysis475(2019)110480
Scheme 1. General outline of the procedure of the Cu-(amino
acid) complex catalysed aerobic oxidative coupling of 2-
naphthol [4].
by washing with ethanol and acetone. Compounds 6 and 7 were already
reported in the literature, but to the best of our knowledge, no reports
exist so far regarding their catalytic activity in the asymmetric oxidative
coupling of 2-naphthol [7,8]. The obtained copper compounds are
water-soluble, but solubility in ethanol is limited. The characterization
of the prepared complexes was carried out by electron paramagnetic
resonance (EPR), electrospray ionization mass spectrometry (ESI-MS)
and by elemental analysis. Circular dichroism (CD), electronic absorp-
tion spectroscopy (UV–vis) were used to assess the spectroscopic
characteristics of the compounds. It was also possible to obtain crystals
suitable for single-crystal X-ray diffraction (XRD) of Cu-complexes 7
and 8. Study of the interaction of 8 with potassium halides was also
carried out by 1H NMR, UV–vis and CD. The redox properties of all Cu
(II) complexes and the effect of potassium iodide on their electro-
chemical behaviour was studied by cyclic voltammetry (CV).
X-ray diffraction studies
The crystal structures of 7 and 8 were established by single crystal
XRD. ORTEP drawings of the asymmetric units of the compounds are
shown in Figs. 1 and 2, respectively, and in Table SD-1 in the Supple-
mentary Data (SD) are listed selected bond lengths and angles.
Compound 7 crystallized in the space group P21, monoclinic system,
displaying two molecules in the asymmetric unit (Fig. 1). Each mole-
cule contains two independent Cu(II) atoms, each coordinated to a
[O,N,O] donor set of the ligand, two water molecules and to a O atom of
a carbonyl moiety of the adjacent [O,N,O] donor set of the ligand,
giving rise to a solid state structure of a coordination polymer (Fig. SD-1
in the SD).
Chart 1. Structural formulas of the ligand precursor compounds prepared and
the corresponding Cu(II) complexes. The letter (S) represents a solvent mole-
cule such as water.
Results and discussion
Synthesis of the ligand precursor compounds
In complex 7, both metal centers can be described as mer (mer-
idional) isomers, as the ligand binds in the meridional coordination
mode, which probably reflects a better stereochemical position of the
ligands. The geometry around the Cu atoms is described as octahedral.
This is indicated by the angles shown by the two trans O- (or N-) atoms
and the metal centre (ca. 180°), and also by the dihedral angles between
the planes of cis-coordinated chelate ligands, which are close to 90°
(Table 1). In each metal centre, two sites are filled by water molecules
(O1 and O11 (for Cu1) and O6 and O12 (for Cu2), respectively), which
bind through intermolecular and intramolecular hydrogen bonds with
the carboxyl O-atoms of the [O,N,O] ligands. Some atoms become
stereogenic centers upon coordination; chiral atoms N1 (Cu 1) and N2
(Cu 2) take the R configuration, whereas atoms C3 (Cu 1) and C10 (Cu
2) retain the S configuration, which is observed in both molecules
present in the unit cell. The bond distances Cu–donor atoms
(1.9439–1.9867 Å) are in the range of those seen for N-, O-donor li-
gands–Cu(II) complexes [9]. The Cu–Owater bond lengths in each Cu
centre (Cu1–O11, 2.6862(15), and Cu2–O12, 2.4625(13) Å) have
longer distances than those displayed in analogous Cu(II) compounds
[9]. A crystal structure similar to that of 7 has been reported by Porter
et al. [8], although some differences between that crystal structure and
the one reported herein were observed. In fact, even though the
monomeric structure is identical in both structures, they crystallized in
the same space group (P21) but with different unit cells
(a = 10.0240(15), b = 7.5434(13), c = 15.151(3) Å, β = 94.815(5)º
for compound 7, and a = 9.840(10), b = 7.260(10), c = 6.630(10),
β = 107.12(4)º for ref. 9, respectively). This is actually due to the ex-
istence of a water molecule co-crystallized in the asymmetric unit of 7,
which is not present in the crystal described in ref. 9.
The ligand precursor compounds were synthesised by N-alkylation
of the sodium salt of the appropriate amino acid with either sodium
chloroacetate or 3-chloropropionate in water, at 95 ºC for 24 h [6,7].
The expected neutral ligand precursors were obtained as white to
cream-coloured precipitates after cooling and acidification of the re-
action mixture with HCl to ca. pH 2. With the exception of the valine
derived compounds, all are insoluble in protic and most aprotic sol-
vents. The insolubility of these compounds was an obstacle for the
adequate characterization by solution NMR. This could be solved by
converting the compounds to the alkali metal salts in situ with in-
organic base such as K2CO3 or KOH in D2O. In contrast, compounds H22
and H24 are very soluble in protic solvents. This, in turn, made their
isolation as pure products much more difficult, as this involved con-
centration of the reaction mixture and careful washing of the obtained
precipitate. In addition, the ligand precursors were characterised by
ESI-MS, using DMSO as sample solvent. The obtained positive and ne-
gative mode MS spectra are included in the Supplementary Data (SD).
The ligand precursors were also characterised by elemental analysis.
Synthesis of the Cu(II)-(amino acid) compounds
The copper compounds were prepared in accordance to the method
published in a previous report [4]: ligand precursor and Cu(OAc)2
monohydrate (OAc = acetate) were suspended in ethanol or water/
ethanol mixture and the resulting suspension was vigorously stirred or
subject to ultrasound irradiation till complete dissolution of the metal
precursor. A blue precipitate formed in all cases. The blue solid was
filtered and the excess Cu(OAc)2 and acetic acid by-products removed
2

P. Adão, et al.
MolecularCatalysis475(2019)110480
Fig. 1. ORTEP-3 diagram of the asymmetric unit of 7, using 30% probability level ellipsoids.
One N- and the two O-atoms (O2 and O3) of the [O,N,O] ligand and one
of the O-atoms of the adjacent carboxylato moiety (O10) define the
equatorial plane, being the metal atom 0.114(6) Å above this plane. A
water molecule (O1) occupies the remaining axial site, with a bond
length of 2.299(9) Å. On the other hand, the coordination sphere of Cu2
is that of an octahedron, the positions being occupied by the two
oxygen and one N-atoms of the ligand (O6, N2, and O7), one water
molecule (O9), and two O-atoms of the acetyl moiety of an adjacent
ligand, forming intramolecular and intermolecular hydrogen bonds. For
both Cu1 and Cu2, all bond distances are in the range of values reported
for related N,O-Cu(II) complexes [8,11]. Also in 8, some atoms become
chiral centers upon coordination; in both molecules of the unit cell,
chiral atoms N1 (Cu 1) and N2 (Cu 2) have the R configuration, and
atoms C2 (Cu 1) and C14 (Cu 2) preserve the S configuration.
The crystal structure of compound 6 has been reported by Nguyen-
Huy Dung et al. [7] but, to the best of our knowledge, no reports of the
crystal structure of 8 are found in the literature.
Mass spectrometry
The ESI-MS spectra (positive and negative mode) of the ligand
precursors (in DMSO) and the corresponding Cu(II) complexes (in a 1:1
mixture of EtOH/H2O) were measured and provided additional struc-
tural information. Most of the obtained spectra are included in the
Supporting Material (Figs. SD-3–SD-22), as well as the assignment
Tables SD-2 and SD-3. For the ligand precursors, the MS spectra are
simple and show the expected molecular ion in most cases. As for the Cu
(II) complexes, extensive aggregation is observed in varying extents in
all spectra. The oligomeric nature of the complexes is particularly evi-
dent in the positive mode MS spectra, from which very limited in-
formation could be extracted in some cases. Despite their complexity,
the negative mode ESI-MS spectra allowed for some peak assignments.
Namely, the negative mode ESI-MS spectrum of 6 (Fig. SD-13) is re-
presentative of this tendency towards aggregation: While monomeric
species are detected, as shown by the peak at m/z = 506 (L = ligand,
[(CuL)(HL)]−), the remaining major species are assigned to mono-
negative dinuclear and trinuclear species such as [(CuL)2(EtOH)]− (m/
z = 615), {[(CuL)2(OH)]·4EtOH·H2O}− (m/z = 790), [(CuL)3(EtOH)]−
Fig. 2. ORTEP-3 diagram of 8, using 30% probability level ellipsoids.
Table 1
Spin Hamiltonian parameters for the prepared Cu(II) compounds.a
Compound gx, gy |Ax|,|Ay| × 104 cm–1 gz
|Az|×104 cm–1 gz/|Az|.cm
6
7
8
9
2.062 13.7
2.061 12.6
2.061 13.7
2.060 13.5
2.059 13.4
2.295 169.8
2.296 171.5
2.287 181.5
2.289 181.1
2.289 180.6
135
134
126
126
127
10
a
The EPR spectra were measured at 77 K in DMSO.
The crystal structure of compound 8 is presented in Fig. 2. It crys-
tallized in the space group P21 within the monoclinic system, with a
molecule in the asymmetric unit. Similarly to 7, the solid state structure
of 8 is that of 1D-coordination polymer (Fig. SD-2 in the SD). In the
asymmetric unit, each molecule contains two independent Cu(II) atoms,
where Cu1 is coordinated to a [O,N,O] ligand, one water molecule (O1)
and to a O-atom (O10) of a carbonyl moiety of the adjacent [O,N,O]
donor set of the ligand, whereas Cu2 is bound to a [O,N,O] donor set,
one water molecule (O9) and to the two O atoms of an acetato group of
the adjacent [O,N,O] donor set of the ligand (O2 and O5). The geometry
displayed at Cu1 is that of a distorted square pyramid (τ = 0.12) [10].
(m/z = 899),
[(CuL)3(EtOH)2(H2O)]−
(m/z = 964),
and
[(CuL)3(HL)]− (m/z = 1076). Another common feature of the negative
mode spectra is the signal at m/z = 247, which could not be assigned to
any formulated fragment, known impurity or polyanionic species.
3

P. Adão, et al.
MolecularCatalysis475(2019)110480
EPR spectroscopy
Table 2
Cyclic voltammetry dataa for Cu(II) complexes obtained in Bu4NBF4/DMSO.
The Cu(II) compounds were examined by low temperature EPR. The
solvent used was DMSO so that aggregation of the complexes upon
sample freezing is minimised. The recorded 1st derivative of the EPR
spectra are presented in Figs. SD-23 and SD-24 (See SD section) Table 1
lists the corresponding spin Hamiltonian parameters. The spectra of 6
and 8 exhibit significant broadening and distortion due to the pro-
nounced tendency of these compounds to aggregate. A secondary spe-
cies may be present in these instances with a z-component g-factor of
ca. 2.2, partially overlapping one on the z-component signals of the
main species.
Epox
Epred
Complex
Ep
(V)
Wave generated by reduction
6
7
8
9
−0.86, −1.10
−1.11, −1.21
−0.96
−0.80
−0.98
−2.03
−2.10
1.02
1.00
1.06
1.13
1.02
0.05(ads); 0.29
0.03(ads); 0.26
0.04; 0.24(ads)
−0.07; 0.22(ads)
−0,04; 0.25(ads)
10
a
Values in V ( 10 mV) versus SCE using Fe(C5H5)2]0/+ (E1o/x2 = 0.44 V) as
internal reference.
Compounds
6 and 7 exhibit very similar z-component spin
Hamiltonian parameters. The N-carboxyethyl variants 8 to 10 show
high similarity with the N-carboxymethyl compounds in all parameters,
with exception of |Az| which is ca. 10 × 10−4 cm−1 higher. Using the
|Az| vs. gz correlation plots and the gz/|Az| tetrahedral distortion index
reported in the literature [12,13], the gz,|Az| representations fall within
the range expected for a Cu(II) centre coordinated to a [N,O] donor
atom set [14,15]. The gz/|Az| index calculated for these compounds is
indicative of some distortion of the square-planar or square-pyramidal
Cu(II) centers, with the N-carboxymethyl compounds being slightly
more distorted than the N-carboxyethyl variants.
Cyclic voltammetry
The redox characteristics of the Cu(II) compounds (6-10) were
studied in DMSO using Bu4NBF4 as electrolyte. Complexes 8 -10 display
by cyclic voltammetry one irreversible cathodic wave attributed to Cu
(II)→Cu(I) reduction, while 6-7 display two successive cathodic pro-
cesses (Waves I and Ia, Fig. SD-30) at potentials that differ by less than
250 mV (Table 2). The two sequential waves are attributed to the re-
duction of two slightly different copper centers in agreement with
structural differences observed by X-ray analysis in the dinuclear
complex 7 (Fig. 1). A third irreversible cathodic process is observed at
potentials lower than the former ones which is attributed to a ligand
based process.
UV-Vis and CD spectroscopy
In contrast, complexes 8-10 display just one irreversible cathodic
process at a potential in between those observed for complexes 6 and 7
(Table 2) suggesting that the two copper centers are electronically si-
milar (see Fig. SD-31).
The UV–vis and CD solution spectra for the Cu(II)-compounds were
also measured and are depicted in Figs. SD-25–SD-28. Tables SD-4 and
SD-5 (see SD section) list the experimental λmax, ε and Δε values. The
electronic absorption spectra of 6 to 10 (Figs. SD-25 and SD-26) are
quite similar, exhibiting a broad band at ca. 700–710 nm. Non-cen-
trosymmetric square-planar or square-pyramidal Cu(II) complexes ty-
pically exhibit these partially allowed d-d transitions in this region
[4,16]. There are some differences in ε of the d-d bands between the N-
carboxymethyl and N-carboxyethyl compounds, and this is evident in
the spectra of 7 and 9. The differences in intensity may be due to the
lower symmetry of 9. Below the 400 nm limit are the intra-ligand or
charge-transfer transitions; the charge transfer bands in 10 are suffi-
ciently intense and broad to reach the 400–500 nm range, thus ex-
plaining the green hue of this complex. The CD spectra of all com-
pounds (Figs. SD-27 and SD-28) exhibit optical activity in the 800-
600 nm range associated with the d-d transitions. This is an indication
of chirality at the Cu(II) center [17]. The N-carboxymethyl compounds
6 and 7 have similar CD patterns (Fig. SD-27), with a single negative
band within the 800-600 nm interval. The CD spectrum of the i-car-
boxyethyl compounds (Fig. SD-28) show a more pronounced optical
activity of the d-d transitions. Only the spectrum of 9 bears some si-
milarity with those of the N-carboxymethyl compounds, in particular
with 7, but even the d-d bands appear to be more intense. Compound 10
exhibits an additional band at ca. 350 nm, which probably corresponds
to charge transfer transitions. The difference in the optical activity of
the N-carboxymethyl and the N-carboxyethyl compounds is clearly seen
in Fig. SD-29 (See SD section), where 6 shows only one negative band in
the 600–800 nm range, while 8 shows a bisignate band within the same
range. This suggests that the conformational effect of the chelate ring
formed by carboxyalkyl pendant arm has an important influence in the
overall chiral-optical properties. It is worth noting that the vicinal and
conformational effects are additive in their contribution to the overall
optical activity [17b]. For instance, the higher torsion of the carbox-
yethyl pendant arm in 8, as seen in the crystal structure, may contribute
to a more pronounced spatial asymmetric distortion when compared to
6, and consequently to a higher splitting of the d-d levels as well as
higher overall optical activity in the 600–800 nm range.
Upon reduction, all complexes display on the reverse scan a char-
acteristic adsorption (Fig. SD-31) attributed to formation of Cu0. The
adsorption process is followed in the cases of complexes 6 and 7 and
preceded in the case of complexes 8–10 by a new anodic wave (A) also
generated by reduction. This data is consistent with two distinct pro-
cesses being promoted by electrochemical reduction, one leading to
formation of a new complex (wave A) and the other to reduction to
copper metal (adsorption). The electrochemical behavior of the N-car-
boxymethyl (6, 7) and N-carboxyethyl (8–10) complexes evidences that
in addition to different structural characteristics, i.e. the six (8–10)
versus the five (6 and 7) member rings of the ONO ligands and distinct
arrangements, the complexes have different electronic properties and
behaviors in electron transfer processes.
In order to check whether the electrochemical behaviour and/or the
redox properties of the complexes changed in the presence of iodide
(known to have a marked effect into the catalytic activity of the com-
plexes) cyclic voltammetry studies were made in the presence of Bu4NI,
chosen to keep the Bu4N+ cation in common with the electrolyte. It is
worth noting that the redox behaviour of the complexes did not change,
neither did the potentials of the cathodic processes, whose values re-
mained within the experimental error ( 10 mV). In the anodic region
oxidation of iodide was clearly observed (Epox = 0.38 V; E1o/x2 = 0.58 V).
New experiments were then set up and KI added to the solution of
complexes 6, 8 and 9, since potassium iodide is the additive used in the
catalytic assays. In this case significant changes were observed in the
cyclic voltammograms, i.e. in the cathodic region wave I defined better,
wave Ia almost disappeared and the potential of wave II (Epred =−1.89)
shifted ca. 120 mV to higher potential (Fig. SD-32). Additionally, the
potential and intensity of the catholically generated waves (adsorption
(*) and anodic (A)) changed as displayed for 6. A shoulder on the left
hand side (b) of the adsorption wave denotes formation of a new species
not observed in the absence of KI (Fig. 3).
The electrochemical behaviour of complexes 8 and 9 in the presence
of KI followed the same trend as 6 showing that KI and Bu4NI have
distinct effects on the redox characteristics of the complexes in solution.
4

P. Adão, et al.
MolecularCatalysis475(2019)110480
whereas under the same conditions the N-carboxyethyl compounds
provide enantiomeric excesses above 40% in all cases (Table 3, entries
8–21). Another important observation is the variability of the conver-
sions of 2-naphthol and BINOL yields for the same conditions. This is
particularly evident for 8 (Table 3, entries 8–17), where conversions
and yields vary between 44% and 73%. This may be due to differing
rates of mass transfer at the air/solvent interface, resulting in differing
rates of diffusion of air into the reaction medium. Since the reactions
were carried out in vessels with vented screw caps, contact with air was
facilitated only by gently stirring the reaction mixture, thus the rate of
diffusion of air is difficult to control under these conditions. No injec-
tion of air into the reaction mixture was made so that solvent eva-
poration is minimised, thus also decreasing changes in the solvent
composition. Solvent composition indeed revealed to be an important
factor and will be addressed in the following section.
Despite this, the overall enantiomeric excesses and selectivity to-
wards the formation of the BINOL product at 40 °C displayed by the N-
carboxymethyl and N-carboxyethyl catalysts remained consistent
throughout. It was also found that carrying out the reactions at 25 °C
instead of 40 °C did not yield improvements regarding selectivity and
enantiomeric excess, but decreased conversion and yields, despite the
longer reaction times. (Table 3, entries 1, 5 and 8).
In face of this, various attempts at optimisation were carried out by
studying the effects of the solvent, cation, anion and basic additive
effect on overall activity of 8, chosen as a model catalyst.
Fig. 3. Cyclic voltammograms of 6 in Bu4NBF4 / DMSO solution (0.1 M), before
(―) and after (—) addition of KI, depicting cathodic generated adsorption (*)
and anodic (A) waves.
In this case, the data obtained by cyclic voltammetry suggests that the
potassium cation plays a significant role in the electrochemical pro-
cesses, thus, in turn, in the catalytic activity (see below).
Asymmetric oxidative coupling of 2-naphthol
Solvent effect
The catalytic activity of compounds 6 to 10 was assessed in the
asymmetric oxidative coupling of 2-naphthol using air (dioxygen) as
oxidant. The results are summarized in Table 3.
The first observation is that the enantiomeric excesses depend
greatly on the N-carboxyalkyl pendant arm length and less on the
amino acid side chain type. For instance, the N-carboxymethyl com-
pounds yield enantiomeric excesses up to 18% (Table 3, entries 1–7)
Given the limited solubility of the complexes in ethanol, a 1:1
ethanol/water mixture was used as solvent in order to facilitate the
dissolution of the catalyst. Considering our previous studies with chiral
N-picolyl-L-amino acid Cu(II) catalysts [4], we anticipated that the ac-
tivity of the present catalysts may also depend on the presence of water.
Therefore, it was necessary to ascertain the optimal ethanol/water ratio
for maximal BINOL yield and enantiomeric excess. Table SD-6 lists the
results obtained with varying ethanol/water ratios.
Table 3
It was observed that the enantiomeric excess decreases significantly
upon increasing the water content of the reaction medium (Fig. 4).
Conversely, selectivity towards the formation of BINOL increases to a
maximum of 74% if the reaction is carried out in water. No clear trend
could be observed relating the content of water with conversion and
yield. Again, the differing diffusion rates of air into the reaction mixture
could explain the fluctuating conversions. The negative impact of water
on enantioselectivity could be due to increased disruption of key aro-
matic π-π and/or cation-π interactions in the asymmetric transition
state. Taking into account this trend, optimal enantioselectivity could
be obtained in 100% ethanol, but selectivity would then decrease since
the BINOL product can also be subject to further oxidative coupling or
oxygenation, as we have observed in previous studies [4]. The low
solubility of BINOL in water may account for the increasing selectivity,
as the product precipitates out of the reaction medium, thus no longer
being available for additional oxidation reactions. In face of the above,
the optimal trade-off between enantioselectivity and product selectivity
was obtained with ethanol/ water mixtures with a water content ran-
ging from 50 to 75%.
Asymmetric oxidative coupling of 2-naphthol using the prepared copper com-
plexes as catalysts and air (dioxygen) as oxidant.a
Entry Catalyst T(ºC) t(h) Conv.(%) Yieldb(%) eeb(%) Selectivity
BINOL(%)
1
2
3
4
5
6
7
8
6
6
6
6
7
7
7
8
8
8
8
8
8
8
8
8
8
9
9
10
10
25
40
40
40
25
40
40
25
40
40
40
40
40
40
40
40
40
40
40
40
40
40
40
72
18
24
24
72
24
24
72
18
24
24
24
24
24
24
24
24
24
24
24
24
24
24
36
41
54
67
31
68
79
28
54
73
69
67
65
63
52
44
42
80
50
75
65
11
12
20
60
34
45
12
45
54
14
31
47
41
36
39
36
35
29
28
51
30
40
37
0
18
16
11
15
16
14
14
52
49
48
44
48
46
47
48
51
48
48
47
40
42
56
73
63
67
39
66
68
50
57
64
59
54
60
57
67
66
62
64
60
53
57
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23c
Very slight differences in performance were observed when ethanol
was replaced with either methanol or isopropanol. With methanol, a
more polar reaction medium is obtained and selectivity towards BINOL
was slightly higher when compared to the isopropanol/water reaction
medium (Table SD-7).
0
a
Reaction conditions: 5 mg of catalyst (ca. 2 mol%), 1 mmol of 2-naphthol,
1 mmol of KI, 4 mL of solvent (EtOH/H2O, 1:1). The reaction mixture was
stirred for 24 h under air at either 25 or 40 °C. A greater volume of solvent
(6 mL) was used in the 72 h runs.
Anion effect
In contrast with what was observed in our previous studies with
chiral N-picolyl-L-amino acid Cu(II) catalysts, all of the Cu(II)-com-
plexes described herein require KI to exhibit optimal catalytic
b
Product yields and enantiomeric excesses were determined by chiral HPLC.
c
Reaction run with 1 mmol of Et3N.
5

P. Adão, et al.
MolecularCatalysis475(2019)110480
Fig. 4. Observed trends in BINOL yields, enantiomeric excesses and selectivity afforded by 8 with increasing water content of the solvent.
performance without added base. The electrode potential of the Cu(II)/
Cu(I) couple is responsive to the presence of halides coordinated to the
copper centre, with the redox couple half-cell potential shifting to more
positive potentials with the coordination of heavier halides [18].
With the Cu(II)-(N-picolyl-L-amino acid) complexes, the [N,N,O]
donor atom set allowed of the Cu(II)/Cu(I) redox couple to remain at a
sufficiently positive electrode potential to induce spontaneous electron
transfer from the 2-naphthol substrate without the assistance of KI,
whereas the more basic N-carboxylalkyl-L-amino acid ligands may be
shifting the electrode potential of the redox couple to increasingly ne-
gative potentials, thus making the aforementioned electron transfer
more difficult [19]. Nevertheless, the effect of halides other than iodide
on the oxidative behavior of 6 and 8 was evaluated and the results are
listed in Table SD-8.
Among the potassium halides tested, only KI is capable of enabling
catalytic activity in the absence of a basic additive. There is some low
catalytic activity when KCl or KBr are chosen as additives. Interestingly,
when KCl is used with catalytic amounts of Et3N, the overall results are
comparable to those obtained with KI alone. This indicates that other
potassium halides may be used additives in place of KI, provided that a
basic additive is also present.
performance when using Li, Na, K and Cs iodides, due to the afore-
mentioned variability of substrate conversions and product yields.
Effect of basic additive
Considering the inactivity of 6 and 8 in the presence of KCl alone
and the results obtained with added base, the effect of the basic additive
was also evaluated. The possibility of these catalysts exhibiting activity
in the absence of iodide salts with only catalytic amounts of Et3N was
also tested. The obtained results are listed in Table SD-10. It was ob-
served that when Et3N was used in catalytic amounts (1 mol%) with
1 mmol of KI, conversions and yields were increased, while maintaining
overall enantiomeric excess and selectivity. Increasing the amount of
Et3N (10–100 mol%) did have a negative effect on selectivity, likely due
to the increased prevalence of over-oxidation reactions. It was also
found that 8 still exhibited catalytic activity when using 1 mol% of Et3N
and KI, or in the absence of KI, although the conversion and yield was
significantly lower, with a slightly lower product selectivity and en-
antiomeric excess. The use of catalytic amounts of morfoline afforded
similar results. However, a large excess of this base exerted a negative
effect on catalyst performance, which may be due to the coordinating
character of morfoline. Using 1 mol% of 2,4,6-collidine did not yield
any apparent improvement over the use of Et3N or morfoline. This
confirms that oxidative coupling with 8 as catalyst is possible with only
catalytic amounts of the tested organic bases, although optimal cata-
lytic performance is only obtained when an excess alkali metal halide,
preferably iodide, is additionally employed.
The amount of KI used in the absence of base is also important. The
catalytic performance of 8 decreased sharply when only 0.1 mmol
(10 mol%) of KI was used instead of 1 mmol. Part of this activity is
regained when 10 mol% of Et3N is also employed, but the overall per-
formance is lower than that obtained when only 1 mmol of KI is used.
The possible reasons for the differences in catalytic activity and its
dependence on the alkali metal iodide additive are addressed in the
following sections.
Elucidation of mechanistic aspects
Cation effect
The mechanism for the oxidative coupling of 2-naphthol using the
Cu(II) catalysts described in this work is expected to be similar to that
proposed earlier in our previous report [4]. The proposed mechanism
consists of the following steps: i) base-assisted binding of 2-naphthol to
the Cu(II) centre, with the proton acceptor being the ligand itself or a
basic additive; ii) one-electron transfer from the coordinated 2-naph-
tholate to the Cu(II) centre, generating a Cu(I)-bonded naphthyloxyl
radical species; iii) homo-coupling of the Cu(I)-bonded naphthyloxyl
radical species to yield a Cu(I)-BINOLate species; iv) product dissocia-
tion and protonation; v) oxidation of the Cu(I) species with atmospheric
O2 and regeneration of the Cu(II) catalytic species. Taking into con-
sideration the catalytic results obtained so far with this system, catalytic
activity is observed when one of the following conditions is fulfilled: i)
an excess of alkali metal iodide is used as additive; ii) a catalytic
amount of an organic base is used as additive.
Not only the anion of the salt additives exerted a determinant effect
on the catalytic performance of 8, it was found that the cation also
appears to play a relevant role in the catalytic process (Table SD-9). It
was observed that considerable catalytic activity is obtained only when
alkali metal iodides are used as additives. Little to no coupling products
were obtained when tetraalkylammonium iodides were used as ad-
ditives, with tetrabutylammonium iodide affording the lowest conver-
sions and BINOL yields. This provides a clearer picture of the role of the
alkali metal halide additive, in particular of the cation, suggesting that
cation-π interactions are also crucial to the catalytic transition state.
Overall, the results suggest a synergistic effect of both the cation and
anion in the modulation of the catalyst activity.
It is worth noting that there is no clear difference in catalyst
6

P. Adão, et al.
MolecularCatalysis475(2019)110480
The role of the basic additive consists in assisting in the binding of
range do occur with all potassium salt additives: there is an increase of
optical activity for 6 and a decrease for 8 in this range, but the bands
associated with the d-d transitions remain essentially unchanged.
Overall, and considering the catalytic results obtained with different
salt additives, this minor change to the CD spectra of 6 and 8 can at-
tributed in part to the interaction of the Cu(II) centre with the halide
anion. While the halide can coordinate directly to the metal centre, the
nature of the interaction of the alkali metal cation (e.g. K+) with the
complex remains unclear.
the substrate to the metal complex, although it was noted earlier that
the overall catalytic activity was much lower when only base was used
compared to the activity obtained when using only KI as additive. It was
found that, when using a basic additive, catalytic activity was enhanced
also when KCl was used as additive. However, little catalytic activity
was obtained when 1 mmol of KCl was used alone. These observations
point to the possible role of the alkali metal halides in the catalytic
cycle. It is known that halides have the effect of shifting the electrode
potential of the Cu(II)/Cu(I) redox couple to more positive values, and
heavier halides exert a stronger effect [18]. Destabilisation of the Cu(II)
oxidation state should occur upon coordination of a halide anion, thus
translating into an enhanced oxidative character of the Cu(II) complex.
When an alkali metal iodide is used, this enhancement is such that the
assistance of a base is no longer necessary for the reaction to take place
to a significant extent. Combination of KI with base accelerates the
reaction further, but may have a negative impact on the overall se-
lectivity.
One noteworthy aspect of the present complexes is the apparent
ionochromism exhibited when in the presence of KI in solution, where
the initially blue aqueous solutions obtained after dissolution of the
complexes turned green upon addition of KI. This indicates that a direct
interaction between the iodide anion and the complex is taking place
[20]. To obtain further clues regarding the effect of these additives on
the catalytic activity of the complexes, UV–vis and CD spectra of 6 and
8 were measured in the presence of KCl, KBr and KI.
It was observed that an additional absorption band appears in the
300–400 nm range (ca. 360 nm) in the UV–vis spectra of both 6 and 8,
when KI is present. No apparent change is observed with KCl or KBr
(Fig. SD-33 for 6 and Fig. 5 for 8). The changes in the visible absorption
spectra observed with KI can be attributed to the halide ligand ex-
change and/or binding in the apical positions of the square-planar or
square-pyramidal Cu(II) centre (See Fig. 5 for compound 8). The dif-
ference in energy between Cu(II)-halide absorption bands could be due
the differences in the HOMO-LUMO gap, with the weaker ligand field-
effect of iodide contributing to a smaller gap. The HOMO-LUMO gaps
for the transitions associated to the Cu(II)-chloride, Cu(II)-bromide
bonds may be wider due to the stronger ligand field effects of chloride
and bromide, thus the respective higher energy absorption bands fall
below the 300 nm limit and are masked by the intra-ligand transitions.
Also of note is the fact that this new absorption band obtained with KI
does not appear in the 300–400 nm range of the CD spectra of 6 and 8
(Figs. SD-34 for 6 and SD-35 for 8). Some changes in the 400–500 nm
Another concern was the possible oxidation of the I− by the Cu(II)
complex. This was evaluated by monitoring the NMR paramagnetic
shifts in the spectra of a sample containing compound 8 and excess of KI
using the Evans method (Fig. 6) [21]. No variation of the MeOH and
tBuOH methyl group shifts was observed after 24 h at 40 °C in a tightly
stoppered NMR tube. No purging of air or inert atmosphere was made
in this case. It can be argued, however, that the presence of air in the
test tube may mask any reduction of the Cu(II) complex by re-oxidation
of the CuI species that might form. To test this possibility, another ex-
periment was made using an NMR tube under a N2 atmosphere.
The methyl group chemical shifts of tBuOH in the 1.2–1.4 ppm
range were considered to calculate the paramagnetic shifts. The ob-
tained paramagnetic shifts for the samples without and with KI were
54.4 Hz and 51.9 Hz, respectively. The paramagnetic shift obtained for
the test run under N2 was also 51.9 Hz. The magnetic moments were
calculated for the square-planar and square-pyramidal species [Cu(4)
(H2O)] (Mw = 316 g/mol) and [Cu(4)(H2O)2] (Mw = 334 g/mol),
which may exist in aqueous solution, given the solution EPR data and
crystal structure obtained for 8 (Table SD-11).
While the magnetic moments obtained for the samples with KI were
slightly smaller, all values are slightly above the magnetic moments for
paramagnetic species with one unpaired electron (1.71 μB) [21g]. This,
in conjunction with the data obtained by cyclic voltammetry, indicates
that oxidation of I− by the Cu(II) complex at 40 °C is unlikely to occur.
To further our understanding of the causes behind the significant
differences in reactivity observed with salt and basic additives, theo-
retical calculations of the spin density distribution were carried out
with a model Cu(II)(iminodiacetate) complex in the presence of 2-
naphthol and iodide.
The spin density distribution in the model complexes [CuI(LH)
(ONAPH)]− (I), [Cu(L)(ONAPH)]− (II) and [Cu(L)(HONAPH] (III)
(L = di(carboxymethyl)amine) calculated using the theoretical (DFT)
methods is shown in Fig. SD-36 (See SD section). In all complexes, spin
density is delocalized between the donor atoms of the ligand L and the
Fig. 5. Isotropic electronic absorption spectra of 8 (2.2 mM), 8 (1.5 mM) with KCl, 8 (1.5 mM) with KBr and 8 (1.7 mM) with KI, recorded at room temperature in
EtOH/H2O 1:1, using 10 mm optical path quartz cells.
7

P. Adão, et al.
MolecularCatalysis475(2019)110480
Fig. 6. 300 MHz 1H NMR spectra of (A) 8 (4.8 mg, 31.3 mM)
in 0.5 mL of MeOH/D2O/tBuOH, (B) 8 (4.7 mg, 30.6 mM) in
the presence of an excess of KI (0.45 M) in 0.5 mL of MeOH/
D2O/tBuOH (1:1:0.2) under air and (C) (4.7 mg, 30.6 mM) in
the presence of an excess of KI (0.45 M) in 0.5 mL of MeOH/
D2O/tBuOH (1:1:0.2) under a N2 atmosphere. All samples
were measured after 24 h at 40 °C in sealed NMR tubes. The
(▪) denotes the sample MeOH methyl group shift, (*) denotes
the reference MeOH methyl group shift, the (•) denotes the
sample tBuOH methyl group shift and (+) denotes the re-
t
ference BuOH methyl group shift.
copper atom. In complex I, the spin density at the copper atom is 0.50 e
indicating that the state of Cu is intermediate between Cu(I) and Cu(II).
Significant spin density is also revealed at the I− ligand (0.17e) and the
ONAPH− ligand (0.19 e), in particular, at the donor O-atom (0.11e) and
one of the α-C-atoms of the naphtyl moiety (0.09 e). In the anionic
complex II, spin density at Cu is noticeably higher (0.58 e) suggesting
that the ligand-to-metal electron transfer is less efficient in II than in I.
Additionally, only the donor O-atom of the ONAPH− ligand has a no-
ticeable spin density (0.12 e). Since, the formation of the naphthyloxyl
radical species is crucial for the efficient coupling of 2-naphthol [4], the
lower overall spin density at the ONAPH− ligand in II compared to I
(0.19 e vs. 0.13 e) may account for the lower reactivity of the Cu(II)
catalysts in the presence of an organic base compared to the addition of
alkali metal iodide. Finally, in the protonated complex III, the overall
spin density at the 2-naphthol ligand is much lower (0.05e) explaining
the inertness of the Cu(II) catalysts in the absence of the additives.
and 7 were significantly less enantioselective than their N-carboxyethyl
counterparts 8 and 9. This contrast is attributed to the factors men-
tioned above in the CD studies. Furthermore, π-π and cation-π inter-
actions play an important role in the asymmetric transition state. This
was particularly evident in the cases where KI and NaI were replaced by
tetraalkylammonium iodides, or when the content of water was in-
creased. The use of an organic base such as Et3N is optional, as long as
catalytic amounts are used. In addition, this also enabled the use of KCl
as inorganic additive, with complex 8 affording comparable results to
those obtained with KI under the same conditions. The role of the base
is that of assisting in the binding of the substrate to the metal centre. An
excess of base leads to lower BINOL yield due to excessive deprotona-
tion of the substrate and its over-oxidation.
The study of the mechanistic aspects revealed that: i) there is an
interaction of KI with the Cu(II) complex, as shown by the apparent
ionochromism and the cyclic voltammetry data, and that ii) the iodide
anion is unlikely to be oxidised by the complexes under the catalytic
conditions used. Computational spin density calculations indicate that
there is an increased spin distribution in the ortho position of the Cu-
bound naphtholate, in contrast to the cases where iodide is absent.
These observations are in line with the outline of the catalytic me-
chanism our group proposed in a previous study [4].
Overall, the series of N-carboxyethyl complexes show promise,
given that enantiomeric excesses surpassing 50% were obtained under
mild reaction conditions and with structurally simple catalysts.
Selectivity towards the formation of BINOL remains at ca. 60% due to
the inherent phenol oxygenase-like character of Cu(II), which needs to
be suppressed more effectively. The improvement of both the yields and
enantiomeric excesses could be achieved by fine-tuning the ligand
structure, where bulky and/or chiral 3-halopropionates are used as
precursors. It is also worth noting that the catalysts operate in low-
toxicity solvent mixtures and mild temperatures, and thus may provide
a contribution to the development of efficient methods for the synthesis
of enantio-enriched BINOLs with low environmental impact.
The advantage of the catalytic system reported herein over the
highly effective and enantioselective copper- and vanadium-based cat-
alysts reported recently in the literature [2,3] lies in the ease of pre-
paration and structural simplicity of the copper complexes, in addition
to the capability of affording enantiomeric excesses of ca. 50% for
BINOL in ethanol/water mixtures. Although there is room for im-
provement, we are confident that the catalytic systems described in this
work present a starting point for the development of a highly en-
antioselective and effective process for the synthesis of enantiopure
BINOLs under mild and low-toxicity conditions.
Conclusions
A range of water-soluble and structurally simple Cu(II) complexes
derived from chiral N-carboxylalkyl amino acids were successfully
synthesised and characterised. Solution and solid state characterisation
indicates a pronounced tendency of these compounds to aggregate and
form oligomeric structures. This is can be observed in the molecular
structures obtained by single crystal X-ray diffraction studies, where
coordination polymers were obtained for 7 and 8. It was also observed
from both the CD and catalytic experiments that the length of the N-
carboxyalkyl pendant arm is a key factor in tuning some of the more
important properties of the complexes, such as optical activity and
overall catalytic activity and/or enantioselectivity. This difference in
the CD profiles can be attributed to the possible conformations of the
chelate rings, with a combination of a 5- and a 6-membered chelate
rings offering a more effective chiral induction than two 5-membered
rings, and/or to a more efficient vicinal effect, as the size of the car-
boxyalkyl pendant arm may contribute to the existence of more rigid
structures and stabilize the chiral-at-the metal diastereoisomeric tran-
sition state that yields higher selectivities [17].
Regarding the acticity of this series of Cu(II)-(amino acid) com-
plexes in the catalytic oxidative coupling of 2-naphthol using air (di-
oxygen) as oxidant, it was found that all operate optimally in 1:1
ethanol/ water mixtures, as long as KI is present in excess. The obtained
results show that activity and enantioselectivity depend on the con-
junction of several factors: i) ligand structure, ii) type of iodide additive
and iii) water content. For instance, The N-carboxymethyl complexes 6
8

P. Adão, et al.
MolecularCatalysis475(2019)110480
Experimental section
same solvent ratio was prepared including 165 mg of KI. Two reference
capillary tubes were prepared using these stock solutions: one reference
for the control tube and the reference with the KI stock for the test tube.
The control NMR tube containing 4.8 mg of 8, was charged with 0.5 mL
of the freshly prepared tBuOH/MeOH/D2O stock solution. The test
NMR tube containing 4.7 mg of 8 was charged with 0.5 mL of the
freshly prepared tBuOH/MeOH/D2O/KI stock solution. The respective
reference capillary tubes were inserted into the NMR sample tubes,
which were then tightly stoppered and vigorously stirred till the com-
plex dissolved completely. The tubes were then heated at 40 °C for 24 h
and the samples analysed by 1H and 13C NMR. The magnetic moments
were estimated using the method developed by Evans et al. modified for
NMR spectrometers with superconducting magnets and applying the
appropriate diamagnetic corrections [21].
Materials and equipment
Chiral amino acids L-phenylalanine (L-Phe) and L-Valine (L-Val) were
purchased from Alfa Aesar and L-tryptophan (L-Trp) was from Panreac.
Chloroacetic acid and 3-chloropropionic acid were from BDH. Metal
precursor copper acetate monohydrate was from Panreac. Solvents
were from Carlo-Erba, Panreac, Sigma-Aldrich or Fisher. All chemical
precursors and solvents were used as received. The UV–vis spectra were
measured on a Shimadzu UV-1603 spectrophotometer and the CD
spectra on a Jasco J-720 spectropolarimeter. The NMR spectra (1H and
13C) were recorded on Bruker Avance+400 MHz and 300 MHz
Spectrometers. 1H and 13C chemical shifts (δ) are expressed in ppm
relative to either NaDSS (sodium 4,4-dimethyl-4-silapentane-1-sulfo-
nate), the deuterated solvent residual peak or, for the 13C spectra, the
carbonate peak. The elemental analyses for all compounds were carried
out at Laboratório de Análises of Instituto Superior Técnico, using an
EA110 CE automatic analyser Instrument. The EPR spectra were mea-
sured with a Bruker EMX X-band spectrometer. The sample for the EPR
measurement were frozen at 77 K and DPPH radical was used as ex-
ternal reference. Simulation of the measured spectra (1st derivative X-
band EPR) was carried out with the computer program developed by
Rockenbauer and Korecz [22]. The cyclic voltammetry studies were
carried out in a three compartment cell provided with platinum wire
electrodes (work and secondary) and a silver wire (reference) electrode,
under an N2 atmosphere. Solutions of Bu4NBF4 in DMSO or MeOH
(0.1 M) were used as electrolytes. The cell was interfaced with a Vol-
taLab PST050 equipment and data acquired using a Pentium(R) Dual-
Core computer. The potentials were measured in Volt ( 10 mV) versus
SCE at 200 mV/s using (Fe(η5-C5H5)2]0/+ as internal reference (DMSO:
X-ray crystallography
Crystals of compounds 7 and 8 were selected, and mounted on a
nylon loop, covered with polyfluoroether oil. Collection of crystal-
lographic data was performed at room temperature, on a Bruker AXS-
KAPPA APEX II diffractometer, using graphite monochromated Mo-Kα
radiation (λ = 0.71073 Å). Bruker SMART software allowed the de-
termination of cell parameters, which were refined using Bruker SAINT
on all observed reflections. The crystal and structure refinement data
for compounds 7 and 8 given in Table SD-12. SADABS was applied for
absorption correction [23]. Direct methods, programs SIR2014 [24]
and SHELXS-97 [25] included in WINGX-Version 2014.1 [26] and
SHELXL, were employed for structure solution and refinement [25]. All
hydrogen atoms were inserted in idealised positions and allowed to
refine riding on the parent carbon atom with C–H distances of 0.93 Å,
0.96 Å, 0.97 Å and 0.98 Å for aromatic, methyl, methylene and methyne
H atoms, and with Uiso(H) = 1.2Ueq(C). Even though the crystals of 8
were of poorer quality, and both complexes were refined as a 2-com-
ponent inversion twin, both refined to a perfect convergence. Graphic
diagrams were prepared with ORTEP-III at the 30% probability level,
and with Mercury CSD 3.9 [27]. Data was deposited in CCDC under the
deposit numbers 1821984 for 7 and 1821985 for 8. Crystal data is listed
in Table SD-12.
red
1/2
red
1/2
E
0.44 V; MeOH:
E
0.43 V). The mass spectrometer used is an ion
trap equipped with an ESI ion source (Thermo Scientific). The equip-
ment was operated in the ESI negative and positive ion modes. The
optimized parameters were as follows: ion spray voltage, + 4/−5.3 kV
; capillary voltage, 5/−20 V; tube lens offset, 63/−124; sheath gas
(N2), 20 arbitrary units; capillary temperature 275 °C. Spectra typically
correspond to the average of 20–30 scans, and were recorded in the
range between 100–2000 Da.
Computational methods
Synthetic procedures
The full geometry optimization of all structures was performed at
the Density Functional Theory (DFT) level with the M06 functional [28]
using the Gaussian-09 package [29]. No symmetry operations were
applied for any of the calculated structures. The relativistic (MDF10) or
quasi-relativistic (MWB46) Stuttgart pseudopotentials were used for the
copper and iodine atoms, respectively, with the appropriate contracted
basis sets (8s7p6d)/[6s5p3d] and (4s5p)/[2s3p] [30]. The basis set for
the iodine atom was augmented by a d-function with exponent 0.243.
The 6-31+G* basis set was used for other atoms. The nature of the
located equilibrium structures was verified by the analytical calcula-
tions of the Hessian matrix (no imaginary frequencies).
The methods for the preparation of all compounds reported herein
are included in the Supplementary Data.
General procedure for the oxidative coupling of 2-naphthol
The solid catalyst (5 mg, ca. 2 mol%), KI (0.01–1.0 mmol) and 2-
naphthol (1.0 mmol) were dissolved in the appropriate solvent (4 mL
for 24–48 h runs, 6 mL for longer runs). The resulting solution was
stirred at either 25 or 40 °C in a glass vessel with a vented screw cap to
maintain a constant contact with air. Control catalytic runs without the
catalyst were alsoncarried out.
The analysis of oxidation profiles was done by chiral HPLC, using a
Daicel Chiralpak IA column and Borwin software. The detection wa-
velength used was 254 nm. The eluent was n-heptane: 1-propanol
(80:20). The flow rate used was 1.0 mL/min. The internal standard
(1 mmol of acetophenone) was added to the reaction mixture before
nanlyis by HPLC. In the case precipitate was present, the reaction
mixtures were homogenised with an appropriate organic solvent.
Acknowledgements
This work was supported by Fundação para a Ciência e a Tecnologia
(FCT/MCTES), UID/QUI/UI0100/2019, the IST-UL Centers of the
Portuguese NMR and Mass Spectrometry Networks (REM2013,
RNNMR, SAICT nº 22125), RECI/QEQ-QIN/0189/2012, RECI/QEQ-
MED/0330/2012, grants SFRH/BPD/107834/2015, SFRH/BPD/
79778/2011 and PD/BD/106078/2015. Pedro Adão acknowledges the
Procedure for the measurement of magnetic moments in solution by NMR
MARE – Marine and Environmental Sciences Centre, Instituto
Politécnico de Leiria, which is financed by national funds from
Fundação para a Ciência e a Tecnologia (FCT/MCTES) (UID/MAR/
04292/2019), the project “SmartBioR- Smart Valorization of
Endogenous Marine Biological Resources Under a Changing Climate”
Two NMR tubes were charged with 4.7 and 4.8 mg of 8, respec-
tively. Separately, a stock solution 200 μL of t-butyl alcohol, 1 mL of
D2O and 1 mL of MeOH prepared. Another stock solution using the
9

P. Adão, et al.
MolecularCatalysis475(2019)110480
(Centro-01-0145-FEDER-000018) co-funded by Centro 2020, Portugal
2020 and European Regional Development Fund (FEDER) and grant
Centro-01-0145-FEDER-000018-BPD4. Clara S. B. Gomes acknowledges
the Associate Laboratory for Green Chemistry- LAQV, which is financed
by national funds from Fundação para a Ciência e a Tecnologia (FCT/
MCTES) (UID/QUI/50006/2019).
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